With the popularity of light-emitting diodes (LEDs), the variety of sizes of bare chips provided by manufacturers also grows. When the size of a bare chip increases, the light output uplifts under a constant current density. However, the heat imposed on the chip also climbs with the increase of input current.
A conventional LED lamp of 5 mm package size has a thermal resistance in the range of 250° C./W˜300° C./W. Provided a high power chip is packaged in a conventional configuration, owing to poor heat dissipation, the surface temperature of the chip is expected to rise rapidly, and the surrounding epoxy is carbonized and discolored. Accordingly, the decay of the chip radiation performance is accelerated till failed. Furthermore, a thermal expansion stress due to a sharp temperature change possibly causes damage to the package structure.
Up to now, the high power LED is generally packaged by forming an insulating material, e.g. epoxy, oxide, and nitride, on a metallic substrate, e.g. Al, and Cu. However, the metallic substrate usually has a thermal expansion coefficient (CTE) greatly different from that of the insulating material. For example, the CTE of Al is about 2.3×10−5/° C., and the CTE of epoxy is about 5˜6×10−5/° C. The CTE of an LED epitaxial material is usually smaller than 1×10−5/° C., or in the range of 4˜8×10−6/° C. For example, the CTE of Al2O3 or AlN is about 4˜8×10−6/° C. At an operation temperature higher than 60° C., the chip easily splits at the interface with the metallic substrate due to the thermal expansion coefficient difference between those materials.
Moreover, the heat crowding in the high-current-driven chip also deteriorates the light output performance of an LED. The silver paste used to fasten the chip has a lower thermal conductivity, and therefore cannot provide a suitable thermal passage for heat transmitting to the metallic substrate. In consequence, the heat from the chip is easily crowded in some regions, which worsen the photoelectric characteristics and the reliability of the chip.